This invention relates to a process for polymerizing, in an aqueous emulsion, ethylenically unsaturated monomers having the group CH.sub.2 =C&lt;, such as, for example, vinyl acetate.
More particularly, this invention relates to a process for continuously polymerizing such monomers, which, on the one hand, does not exhibit the shortcomings of the processes of the prior art that will be described hereinafter, and, on the other hand, provides aqueous dispersions of polymers having constant analytical and chemical-physical characteristics.
It is known that aqueous dispersions of polymers of ethylenically unsaturated monomers can be prepared according to either a discontinuous (i.e., batchwise) or a continuous process.
In the commercial production of such dispersions by means of the discontinuous process, it is generally necessary to employ very large batches of materials which present difficulties, such as that of keeping a certain uniformity of the product among the large batches of materials and that of maintaining the temperature required by the process, the latter difficulty being the greater, the larger the volume of the equipment used.
Conversely, the prior art continuous processes allow for easier operating conditions and offer several distinct advantages, such as, for instance, better quality of the product, higher productivity, use of smaller reactors, and lesser expenses, in comparison with operating discontinuously; furthermore, by controlling the process as automatically as possible, products exhibiting a more homogeneous composition can be obtained.
The prior art continuous polymerization processes in emulsion are of two types:
(a) wherein the polymerization is conducted in a single reactor, generally cylindrical, in which the various polymerization steps take place in different sections of the reactor; and PA1 (b) wherein the polymerization is conducted in several reactors connected with one another in series and arranged at heights decreasing from the first reactor to the last one, each of such reactors having to carry out a particular polymerization step. PA1 (1) Polymerization in a first reactor (seeding pot) of an aqueous pre-emulsion -- added continuously and made up of a monomer portion (5- 20% of the total monomer) and of all the other ingredients (water, initiators, surfactants, buffers, etc.); PA1 (2) transferring the resulting dispersion to a second reactor (polymerizing pot), to which the remaining monomer amount (95-80%) is continuously fed; PA1 (3) transferring the dispersion from the polymerizing pot to a third reactor (finishing pot), in which the polymerization is completed; PA1 (4) discharging the finished product through a cooler, for instance of the plate or coil type.
The processes of type (a) utilize apparatuses equipped with particular devices suited to remove the troubles that may occur during the polymerization, such as the formation of scales on the reactor walls due to the lack of stirring capability or to superheating.
Conversely, the processes of type (b) do not require any special apparatuses. That is, it is enough to utilize two or more conventional reactors connected in series or connected with one another by means of pumps capable of transferring the mass being polymerized or the finished product.
Furthermore, the processes of type (a) do not provide satisfactory results, particularly with regard to the characteristics of the dispersions, which depend upon the particle sizes of the polymer produced. For example, it is a fact that particles exhibiting an extraordinarily large diameter are obtained when a single phase comprising monomer, initiator, surfactant, protecting agent and buffer, is fed to the reactor. A high value of the particles' average diameter results in mechanical instability and a low viscosity of the dispersions, thus causing sedimentation and degradation of the final product.
Such shortcomings are generally eliminated by effecting the polymerization according to a process of type (b) and employing a particular distribution technique of the reactants in different polymerization steps, operating with a number of reactors connected in series. Such technique is based on the following operations:
In accordance with such a technique, a high emulsifier/monomer ratio, like that existing in the first reactor, brings about a "nucleating" step characterized in that several polymerization nuclei form, which subsequently, in the second reactor, give rise to real polymer particles having a relatively small diameter. In fact, since operative conditions in the polymerizing pot are different and the surfactant/monomer ratio is lower, it is impossible to cause the formation of new nuclei, but only to cause them to grow due to the presence of larger amounts of monomers.
If the feeding conditions in the first two reactors are kept constant, a steady state is attained, i.e., a reaction condition is attained in which the composition of the reacting mixture remains constant, as long as the feeding of the various components and the composition of the resulting mixture, the level of the reacting masses in the various reactors and the discharge of the final product -- which in this way assumes a well-defined and homogeneous composition in the long run -- remain constant.
The continuous emulsion polymerization of ethylenically unsaturated monomers in several reactors connected in series is described in Italian Pat. No. 366,073 (I. G. Farbenindustrie) and in U.S. Pat. No. 2,587,562 (Shawinigan Resins Corp.). Furthermore, Belgian Pat. No. 652,607 (Shell Internationale Research Maatschappij) describes a particular process of this type, in which the first polymerization step is conducted by keeping a high emulsifier/monomer ratio with a view towards promoting nucleation.
In these processes of type (b) described in the above-mentioned patents, the transfer of the reaction mixture from one reactor to the other occurs through overflow devices proceeding from the reactor located at the highest level to the one located at the lowest level. This technique results in serious problems. In particular, by operating in this way, the mass subjected to polymerization usually takes up a more or less considerable fraction of the volume of each reactor depending upon the point of the lateral outlet. Under these conditions, owing to even slight variations in the polymerization kinetics due, for example, to the difficulty of accurately controlling the inside temperatures or the feeding flow rates, foamings and swellings of the reaction mass often occur, which unavoidably cause the forming of hard and thick scales of polymer on the reactor walls over the dispersion withdrawing level, on the reactor dome, near the dispersion outlet, on the inside surface of the conveying pipes and on the non-immersed part of the shaft of each stirrer. The extent of the scales which form may be such as to result in the termination of the operation and other serious problems, such as variation of the final product characteristics, particularly relating to the particle distribution and their diameter size, the viscosity and stability of the dispersion, the conversion of the monomer to polymer, all of these parameters depending upon the residence time of the mass inside the reactor, which necessarily varies due to the phenomena cited hereinbefore, and being further badly affected by local superheatings due to insufficient heat exchange with the outside.